The present invention relates to a method of synchronising a radio transmitter with other receivers in a network, in order to minimise the effect of interference from other transmitters. In particular, the invention relates to the synchronisation of a network of base stations in a mobile communications system.
In a digital cellular radio system which operates in accordance with the TDMA-principle, such as GSM, D-AMPS or PDC, radio messages are transmitted in frames from base stations, each frame including a given number of time slots. Transmissions in the different time slots are generally intended to be received by different mobile radio receivers, and so it is necessary to ensure that the receiver is synchronised with the transmitter. The mobile transceivers are synchronised to their respective base stations by a signal from the base station.
For example, in full-rate D-AMPS, three mobile receivers share the same frequency channel, and therefore each channel is divided into three time slots, each lasting 6.7 ms, three time slots forming a frame. The frames are repeated 50 times per second.
Each time slot in the frame is assigned to a particular mobile receiver, until either the call is released, or the mobile is handed over to another channel, for example in another cell.
In each time slot, 324 bits are transmitted, most of which are data bits, but 28 of which form a synchronisation word.
The standard published by the Electronics Industries Association as EIA/TIA Publication IS136, which specifies the D-AMPS system, defines six different synchronisation words, but only three of those are used for a channel running at full-rate. Thus, a different synchronisation word is allocated to each time slot in a frame, and the base station transmits the relevant synchronisation word once during each time slot. The mobile receiver is able to recognise transmissions intended for it by identifying the synchronisation word, and similarly includes the same synchronisation word in its own transmissions to the base station.
The synchronisation words in IS136 are chosen such that there is minimal correlation between them. Thus, there is only a very small chance that a receiver will mis-identify a transmitted synchronisation word with a different synchronisation word. A danger, however, is that a receiver will receive the expected synchronisation word from an interfering transmitter operating on the same frequency, and will falsely interpret it as its expected synchronisation word. Moreover, there is the possibility that a receiver will falsely interpret data sent by an interfering transmitter on the same frequency as its expected synchronisation word.
Attempts have been made in the prior art to overcome these problems.
One known possibility is simply to allow each base station transceiver to select its own timing, meaning that there is no synchronisation between the base stations. In this situation, it is possible, although rather unlikely, that an interfering transmitter will be transmitting the same synchronisation word with a signal level sufficient to cause interference, and at a point in time sufficiently close to the expected time to give the possibility of false synchronisation.
An alternative known possibility is to synchronise the entire network, such that every base station is transmitting the same synchronisation word at the same time. This increases the probability that it will falsely receive the synchronisation word from an interfering transmitter, and interpret it as its own expected synchronisation word.
The probability of a false identification in this way depends on the carrier/interference (C/I) ratio, which relates to the relative signal levels of the transmissions from a desired transmitter, and from an interfering transmitter operating on the same frequency. Because there are only a limited number of frequencies available for use in a system, it is necessary to re-use the frequencies. Frequency planning can maximise the frequency re-use distance, and hence maximise the C/I ratio, but in general cannot guarantee that the C/I ratio will be high enough to avoid any possibility of interference by falsely detecting the synchronisation word from an interfering transmitter.
It is an object of the invention to increase the probability of correct synchronisation, by planning the use of the synchronisation words.
A network in accordance with the invention comprises a number of base stations, some of which use the same frequency. In accordance with the invention, the base stations are synchronised, so that their time slots coincide, and the network is planned such that the neighbouring base stations operating on the same frequency transmit different synchronisation words during any given time slot.
The invention also relates to the method of coordination of the air frame positions of the base stations, and to the base stations themselves.
The advantage of the invention is that the risk of false synchronisation can be minimised, even under less favourable C/I conditions.